Heliostat calibration

ABSTRACT

Embodiments relate to solar energy systems and methods of operating the same. In some embodiments, the solar energy system comprising: a plurality of heliostats configured to reflect sunlight to a target mounted on a tower, each heliostat including a respective heliostat controller, the target, the target being selecting from the group consisting of an energy conversion target and/or a secondary reflector; and a macro-array of light-intensity sensors characterized by a maximum sensor-sensor distance and mounted on the tower such that when any heliostat of the plurality of heliostats reflects a beam of light onto the macro-array of light-intensity sensors, the maximum dimension of the reflected beam&#39;s projection on the macro-array is at most twice the maximum sensor-sensor distance, wherein each heliostat controller is operative to control its respective heliostat so that the light beam reflected by the heliostat traverses the macro-array of light-intensity sensors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication No. 61/109,205 filed on Oct. 29, 2008.

FIELD OF THE INVENTION

Embodiments relate to the conversion of solar radiation to usable formsof energy such as heat or electricity.

SUMMARY OF EMBODIMENTS

Some embodiments of the present invention relate to systems, methods andarticles of manufacturing for facilitating the conversion of solarradiation to thermal and electric energy.

Embodiments of the present invention relate to a solar energy systemcomprising: (a) a plurality of heliostats configured to reflect sunlightto a target mounted on a tower, each heliostat including a respectiveheliostat controller, the target, the target being selecting from thegroup consisting of an energy conversion target and/or a secondaryreflector; and (b) a macro-array of light-intensity sensorscharacterized by a maximum sensor-sensor distance and mounted on thetower such that when any heliostat of the plurality of heliostatsreflects a beam of light onto the macro-array of light-intensitysensors, the maximum dimension of the reflected beam's projection on themacro-array is at most twice the maximum sensor-sensor distance, whereineach heliostat controller is operative to control its respectiveheliostat so that the light beam reflected by the heliostat traversesthe macro-array of light-intensity sensors.

In some embodiments, the macro-array is substantially co-planar.

In some embodiments, the macro-array of light-intensity sensors is a twodimensional macro-array.

In some embodiments, the light-intensity sensors are configured toacquire time-series light intensity data while the projected light beamtraverses across the macro-array of light sensors.

In some embodiments, the heliostat controller is operative to: i) beforethe beam traversing, direct the heliostat to the target mounted on thetower according to an initial set of aiming parameters; and ii) afterthe beam traversing, redirect the heliostat to the target mounted on thetower according to a modified set of aiming parameters that is modifiedin accordance with light intensity data generated by light intensitysensors of the macro-array.

In some embodiments, the heliostat controller is operative to effect there-directing according to the modified set of aiming parameters afterthe beam traversing.

In some embodiments, the heliostat controller is operative to effect there-directing according to the modified set of aiming parametersimmediately only after a time delay.

In some embodiments, during the period of the time delay, the controlleris operative to re-direct the heliostat to the target according to theinitial set of aiming parameters.

In some embodiments, the modified set of aiming parameters is modifiedin accordance with at least one of: i) distances between light-intensitysensors of the macro-array of light-intensity sensors; ii) a beamtraversal speed of the traversing reflected heliostat beam.

In some embodiments, the system further comprises: c. a heliostat-fieldcontroller operative to: i) select, from the plurality of heliostats, asub-plurality of heliostats that is to be simultaneously directed to thetarget (i.e., operated so that at least one point in time they are alldirected to the target—there is no requirement to simultaneouslyre-orient the heliostats of the sub-plurality); and ii) direct theselected sub-plurality of heliostats the target (i.e., cause a situationwhere the selected sub-plurality is simultaneously directed to thetarget), wherein the heliostat field controller is operative to carryout the heliostat selection in accordance with respective lightintensity measurements of macro-array taken when each heliostat'sreflected beam respectively traverses the macro-array.

In some embodiments, only the selected sub-plurality is directed to thetarget. Alternatively, at least the selected sub-plurality is directedto the target.

In some embodiments, the heliostat-field controller is operative toeffect the selection in accordance with at least one of: i) distancesbetween light-intensity sensors of the macro-array of light-intensitysensors; and ii) a beam traversal speed of the traversing reflectedheliostat beam.

In some embodiments, the system further comprises: c) electroniccircuitry configured to measure at least one beam projection parameterof the heliostat beam according to the light intensity measurementsacquired by light-intensity sensors while the heliostat beam traversesthe macro-array of light-intensity sensors.

In some embodiments, the electronic circuitry is configured to effectthe measuring in accordance with at least one of: i) distances betweenlight-intensity sensors of the macro-array of light-intensity sensors;and ii) a beam traversal speed of the traversing reflected heliostatbeam.

In some embodiments, the system further comprises: c) electroniccircuitry configured to measure at least one of: i) a shape of theheliostat beam; ii) a flux intensity map of the heliostat beam; and iii)an offset of the heliostat beam, according to the light intensitymeasurements acquired by light-intensity sensors while the heliostatbeam traverses the macro-array of light-intensity sensors.

In some embodiments, the electronic circuitry is configured to effectthe measuring in accordance with at least one of: i) distances betweenlight-intensity sensors of the macro-array of light-intensity sensors;and ii) a beam traversal speed of the traversing reflected heliostatbeam.

In some embodiments, the heliostat controllers collectively areconfigured so that multiple overlapping heliostat reflection beamsincluding first and second heliostat reflection beams simultaneouslytraverse the macro-array to simultaneously illuminate one or more of thelight-intensity sensors.

In some embodiments, i) the light-intensity sensors of the macro-arrayare image sensors; and ii) the system further comprises: c) electroniccircuitry operative to: A) determine, from the images generated by theimage sensors, relative light intensity contributions of the overlappingfirst and second heliostat beams when the first and second beams overlapand traverse the macro-array; and B) in accordance with the relativelight intensity contributions, determine at least one of: I) a shape ofthe first and/or second heliostat beam; II) a flux intensity map of thefirst and/or second heliostat beam; and III) an offset of the firstand/or second heliostat beam.

In some embodiments, the heliostat controllers collectively areconfigured so that the first and second heliostat beams overlap at sometimes and are disjoint at other times while the first and second beamstraverse the macro-array.

In some embodiments, i) the light-intensity sensors of the macro-arrayare image sensors; and ii) the system further comprises: c) electroniccircuitry operative to determine when the first and second beams aredisjoint, and in accordance with the disjoint time period(s), determineat least one of: I) a shape of the first and/or second heliostat beam;II) a flux intensity map of the first and/or second heliostat beam; III)an offset of the first and/or second heliostat beam; and IV) anindication of beam area (for example, beam diameter or any otherindication).

In some embodiments, each of the light sensors of the macro-array areimage sensors.

In some embodiments, the image sensors are selected from the groupconsisting of a CCD microarray and a CMOS microarray.

In some embodiments, each of the light sensors of the macro-array arephoto-detectors incapable of detecting an image.

In some embodiments, each of the light sensors are photo-voltaic cells.

In some embodiments, each of the light-intensity sensors is mounted tothe tower.

In some embodiments, the energy conversion target is selected from thegroup consisting of solar boiler target and a molten salt solarreceiver.

In some embodiments, the solar boiler target is selected from the groupconsisting of a solar evaporator, a solar re-heater and a solarsuperheater.

In some embodiments, the energy conversion target includes one or morephotovoltaic and/or photo-electrovoltaic cells.

In some embodiments, the tower height is at least 25 meters.

In some embodiments, the tower height is at least 100 meters.

In some embodiments, the system further comprises: c. a projectorconfigured to project artificial light onto the heliostat such that thetraversing reflected beam that traverses the macro-array includes theartificial light generated by the projector.

In one example, the projector is configured so that the apparent width(i.e., either as detectable at the heliostat mirror or at themacro-array of light sensors) of the light source of the projector isequivalent to the apparent width from the sun.

In some embodiments, the projector is mounted on the tower.

Some embodiments of the present invention provide a method of operatinga solar energy system, the method comprising:

a. reflecting sunlight from each of a plurality of heliostats to atarget mounted on a tower, the target being selecting from the groupconsisting of an energy conversion target and/or a secondary reflector;and

b. respectively controlling each heliostat of the plurality so that thelight beam reflected by the heliostat traverses the macro-array oflight-intensity sensors characterized by a maximum sensor-sensordistance and mounted on the tower such that when any heliostat of theplurality of heliostats reflects a beam of light onto the macro-array oflight-intensity sensors, the maximum dimension of the reflected beam'sprojection on the macro-array is at most twice the maximum sensor-sensordistance.

In some embodiments, time-series light intensity data is acquired by thelight-intensity sensors while the projected light beam traverses acrossthe macro-array of light sensors.

In some embodiments, the method further comprises: before the beamtraversing, directing the heliostat to the target mounted on the toweraccording to an initial set of aiming parameters; and after the beamtraversing, re-directing the heliostat to the target mounted on thetower according to a modified set of aiming parameters that is modifiedin accordance with light intensity data generated by light intensitysensors of the macro-array.

In some embodiments, the re-directing is carried out according to themodified set of aiming parameters after the beam traversing.

In some embodiments, the re-directing is carried out according to themodified set of aiming parameters immediately only after a time delay.

In some embodiments, the re-directing the heliostat to the target iscarried out according to the initial set of aiming parameters.

In some embodiments, the modified set of aiming parameters is modifiedin accordance with at least one of: i) distances between light-intensitysensors of the macro-array of light-intensity sensors; ii) a beamtraversal speed of the traversing reflected heliostat beam.

In some embodiments, the method further comprises: i) selecting, fromthe plurality of heliostats, a sub-plurality of heliostats that is to besimultaneously directed to the target (i.e., operated so that at leastone point in time they are all directed to the target—there is norequirement to simultaneously re-orient the heliostats of thesub-plurality); and ii) directing the selected sub-plurality ofheliostats the target (i.e., cause a situation where the selectedsub-plurality is simultaneously directed to the target), wherein theheliostat selection is carried out in accordance with respective lightintensity measurements of macro-array taken when each heliostat'sreflected beam respectively traverses the macro-array.

In some embodiments, only the selected sub-plurality is directed to thetarget. Alternatively, at least the selected sub-plurality is directedto the target.

In some embodiments, the selection is carried out in accordance with atleast one of: i) distances between light-intensity sensors of themacro-array of light-intensity sensors; and ii) a beam traversal speedof the traversing reflected heliostat beam.

In some embodiments, at least one beam projection parameter of theheliostat beam is measured according to the light intensity measurementsacquired by light-intensity sensors while the heliostat beam traversesthe macro-array of light-intensity sensors.

In some embodiments, the measuring is carried out in accordance with atleast one of: i) distances between light-intensity sensors of themacro-array of light-intensity sensors; and ii) a beam traversal speedof the traversing reflected heliostat beam.

In some embodiments, at least one of the following is measured: i) ashape of the heliostat beam; ii) a flux intensity map of the heliostatbeam; and iii) an offset of the heliostat beam, according to the lightintensity measurements acquired by light-intensity sensors while theheliostat beam traverses the macro-array of light-intensity sensors.

In some embodiments, the measuring is carried out in accordance with atleast one of: i) distances between light-intensity sensors of themacro-array of light-intensity sensors; and ii) a beam traversal speedof the traversing reflected heliostat beam.

In some embodiments, multiple overlapping heliostat reflection beamsincluding first and second heliostat reflection beams simultaneouslytraverse the macro-array to simultaneously illuminate one or more of thelight-intensity sensors.

In some embodiments, the method further comprises: A) determining, fromthe images generated by the image sensors, relative light intensitycontributions of the overlapping first and second heliostat beams whenthe first and second beams overlap and traverse the macro-array; and B)in accordance with the relative light intensity contributions, determineat least one of: I) a shape of the first and/or second heliostat beam;II) a flux intensity map of the first and/or second heliostat beam; andIII) an offset of the first and/or second heliostat beam.

In some embodiments, the first and second heliostat beams overlap atsome times and are disjoint at other times while the first and secondbeams traverse the macro-array.

In some embodiments, i) the light-intensity sensors of the macro-arrayare image sensors; and ii) the method further comprises: determining atleast one of: I) a shape of the first and/or second heliostat beam; II)a flux intensity map of the first and/or second heliostat beam; III) anoffset of the first and/or second heliostat beam; and IV) an indicationof beam area (for example, beam diameter or any other indication).

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrammatic elevation views of a plurality ofheliostats and a central power tower in accordance with differentembodiments of the invention.

FIG. 3 is a block diagram of a hierarchical control system for a solarpower tower system.

FIGS. 4A-6B are diagrammatic elevation views of various examples ofarrays of light intensity detectors.

FIGS. 7A-7C are flowcharts of routines of operating a solar energysystem.

FIG. 8 illustrates heliostat offset.

FIGS. 9A and 9B are diagrammatic elevation views of a heliostat and acentral power tower system equipped with an array of light intensitydetectors.

FIG. 10 is a diagrammatic elevation view of a heliostat and a centralpower tower system equipped with an array of light intensity detectorsand an additional mechanical element in accordance with a preferredembodiment.

FIG. 11 is a diagrammatic plan view of a solar power tower systemshowing an example of how a plurality arrays can be provided to cover asurround heliostat field.

FIG. 12 a diagrammatic elevation view of a heliostat and a central powertower system equipped with an array of light intensity detectors and alight projector in accordance with a preferred embodiment.

FIG. 13 is diagrammatic elevation view of an array of light intensitydetectors illustrating a method for calibrating multiple heliostats atsubstantially the same time.

FIGS. 14A-14F illustrate time series of multiple beams traversing anarray of light intensity sensors in accordance with some embodiments.

FIGS. 15A, 16 are flowcharts of routines for characterizing reflectionbeams of one or more heliostats.

FIG. 15B illustrates multiple heliostats directed to a macro-array oflight intensity sensors.

FIG. 16 illustrates an image of at least a portion of a field ofheliostats.

DETAILED DESCRIPTION OF EMBODIMENTS

According to the some embodiments, a solar power tower system includesat least one tower and at least one set of heliostats. Each heliostattracks to reflect light to a target on a tower. The heliostats can bearrayed in any suitable manner, but preferably their spacing andpositioning are selected to provide optimal financial return over a lifecycle according to predictive weather data and at least one optimizationgoal such as total solar energy utilization, energy storage, electricityproduction, or revenue generation from sales of electricity.

An ‘energy conversion target’ or solar receiver uses reflected andoptionally concentrated solar radiation and converts it to some usefulform of energy, such as heat or electricity. The solar receiver may belocated at the top of a receiver tower or at some other location, forexample if an intermediate reflector (also called a secondary reflector)is used to bounce light received at the top of a tower down to areceiver located at ground level or at an intermediate height. For thepresent disclosure, the terms ‘energy conversion target’ and ‘solarreceiver’ are used interchangeably and refer to a device or apparatusfor converting insolation into some other form of energy—for example,electricity or thermal energy.

Referring now to the figures and in particular to FIG. 1, a solar powertower system 44 is provided in which heliostats 38 include mirrors 8that reflect incident solar radiation 28 onto a target mounted on tower44 (for example, solar receiver 1). The heliostat-mounted mirrors 8 arecapable of tracking the apparent movement of the sun 25 across the skyeach day in order to maintain the reflective focus in the direction ofthe receiver 1 as the angle of the incident radiation 28 changes. Thistracking capability may be provided at least in part by a heliostatcontroller (not shown in FIG. 1—see, for example, element 65 of FIG. 3)for controlling one or more orientation parameters of mirror 8 to aimreflection beam 398.

The skilled artisan will realize that the heliostat controller mayinclude any combination of mechanical parts (for example includingmotors, actuators, etc) and/or electrical circuitry (for example,integrated circuits). In one non-limiting example, the electricalcircuitry includes one or more computer microprocessors configured toexecute software or code module(s) residing in volatile memory. Inanother non-limiting example, the heliostat controller may include gatearray electronics for example, field-programmable gate array (FPGA). Aswill be explained below, in some embodiments, heliostat controller ofheliostat 38 may also be configured for aiming the mirrors 8 atlocations other than the target located atop tower 43.

In the example of FIG. 1, solar receiver 1 (for the present disclosure,“receiver” and “solar receiver” are used interchangeably) is locatedatop a tower 43. FIG. 2 illustrates an alternative embodiment wherereceiver 1 is located on the ground, and the target is a secondaryreflector (in contrast to FIG. 1 where the target is a solar receiver).Thus, in the example of FIG. 2, the heliostat-mounted mirrors 8 reflectsolar radiation onto one or more secondary reflectors 9 which furtherreflect the radiation onto the receiver 1.

As shown in the figures, the reflection of the incident radiation beam28 produces a reflection beam 398 which is reflected to the target,which is either a solar receiver (i.e., for converting insolation toanother form of energy such as thermal energy or electricity) or asecondary reflector configured to relay light to a solar receiver. Inone example, the solar receiver (either mounted on the tower as in FIG.1 or operative to receive insolation from a secondary mounted on thetower as in FIG. 2) is a solar boiler for boiling water and/or heatingsteam—this solar boiler may be operatively linked to an apparatus 45 forconverting solar steam to electricity. In another example, the solarreceiver is a molten salt solar receiver. In yet another example, thesolar receiver includes one or more ‘efficient’ photovoltaic and/orphotoelectrochemical cells.

For the present disclosure, ‘efficient’ photovoltaic and/orphotoelectrochemical cell are cells capable of effecting ‘efficient’(i.e., at least 10% efficiency) photovoltaic conversion atconcentrations of at least 20 suns. Photovoltaic and/orphotoelectrochemical cell that are unable to reach this level ofefficiency are referred to as ‘inefficient’ cells.

It is appreciated that FIG. 1 (and none of the figures) is not requiredto be to scale—for example, in some embodiment, tower is much taller(e.g. at least 5 times or 10 times or 20 times or more) than heliostats38. In one example, the tower height is at least 25 meters. In anotherexample, the tower height is at least 100 meters.

Although the heliostat mirror is drawn in the figs as a straight linerepresenting a planar mirror, it is appreciated that this is not alimitation, and that other shaped mirrors may be employed.

As noted above, each heliostat may include a heliostat controller (NOTSHOWN) including mechanical parts and electrical circuitry for trackingthe sun. In some embodiments and as will be discussed below, heliostatcontrollers may be operative to move the reflection beam to anotherlocation other than the target (i.e., the receiver 1 or the reflector9). One example of such a target is the ‘macro-array’ of light-intensitydetectors discussed below.

In some embodiments, each heliostat controller is autonomous and may aimmirror 8 to provide a certain functionality without requiring externalinput. Alternatively or additionally, each heliostat controller mayrespond to one or more electronic communications (for example, externalcommands) received from external electronic device or system (located atany location) describing how to aim mirror 8. For both cases, it may besaid that the heliostat controller is ‘operative’ to provide thefunctionality (for example, aiming functionality).

A solar power tower system 44 also generally includes a heliostat fieldcontrol system (not shown in FIGS. 1-2) for helping the system operatoror owner attain or maintain pre-defined operating parameters and/orconstraints, some of which may be based on achieving optimization goalsand some of which may be based on maintaining the safety of the systemand its operation. For example, a heliostat field control system can beused to ensure that light energy flux is distributed across the surfaceof a target in accordance with a predetermined set of desired values(see for example, WO/2009/103077 incorporated herein by reference in itsentirety), or it can be used to maximize conversion of energy from solarradiation to latent and/or sensible heat in a working fluid within areceiver, and/or conversion of solar energy to electricity byphotovoltaic (or photoelectrochemical) means, while ensuring that localtemperatures on the surface of the receiver, or local concentrations ofsolar flux, do not exceed a predetermined local maximum.

Overall control of the multiple heliostats can be either centralized ina single computer or distributed among several or many processors. Thus,in some embodiments, decisions about where to aim the heliostats may becarried out locally by the various heliostat controllers. Alternativelyor additionally, heliostat field controller may communicate aiminginstructions to one or more heliostat controllers which are configuredto then provide this aiming functionality.

In an exemplary embodiment, a central heliostat field control systemcommunicates hierarchically through a data communications network withcontrollers of individual heliostats. FIG. 3 illustrates an example ofsuch a hierarchical control system 91 that includes three levels ofcontrol hierarchy, although in other embodiments there can be more orfewer levels of hierarchy, and in still other embodiments the entiredata communications network can be without hierarchy, for example in adistributed processing arrangement using a peer-to-peer communicationsprotocol.

At a lowest level of control hierarchy (i.e., the level provided byheliostat controller) in the illustration there are providedprogrammable heliostat control systems (HCS) 65, which control thetwo-axis (azimuth and elevation) movements of heliostats (not shown),for example as they track the movement of the sun. At a higher level ofcontrol hierarchy, heliostat array control systems (HACS) 92,93 areprovided, each of which controls the operation of heliostats 38 inheliostat fields 96,97 respectively, by communicating with programmableheliostat control systems 65 associated with those heliostats 38 througha multipoint data network 94 employing a network operating system suchas CAN, Devicenet, Ethernet, or the like. At a still higher level ofcontrol hierarchy a master control system (MCS) 95 is provided whichindirectly controls the operation of heliostats in heliostat fields96,97 by communicating with heliostat array control systems 92,93through network 94. Master control system 95 further controls theoperation of a solar receiver (not shown) by communication throughnetwork 94 to a receiver control system (RCS) 99. In the exampleillustrated in the figure, the portion of network 94 provided inheliostat field 96 is based on copper wire or fiber optics connections,and each of the programmable heliostat control systems 65 provided inheliostat field 96 is equipped with a wired communications adapter 76,as are master control system 95, heliostat array control system 92 andwired network control bus router 100, which is optionally deployed innetwork 94 to handle communications traffic to and among theprogrammable heliostat control systems 65 in heliostat field 96 moreefficiently. In addition, the programmable heliostat control systems 65provided in heliostat field 97 communicate with heliostat array controlsystem 93 through network 94 by means of wireless communications. Tothis end, each of the programmable heliostat control systems 65 inheliostat field 97 is equipped with a wireless communications adapter77, as are heliostat array control system 93 and wireless network router101, which is optionally deployed in network 94 to handle networktraffic to and among the programmable heliostat control systems 65 inheliostat field 97 more efficiently. In addition, master control system95 is optionally equipped with a wireless communications adapter (notshown).

One of the possible functions of a control system (including localheliostat controller(s) and/or one or more higher-level controllers—forexample, a centralized heliostat field controller) is to directheliostats to various aiming points on the surface of a target, oralternatively not on the surface of a target when operating conditionsrequire it. This is done on the basis of periodically or continuouslyevaluating various inputs, which can include (but not exhaustively):predictive and/or measured meteorological data; and measured and/orcalculated operating conditions and parameters of heliostats andreceivers. Among the operating conditions and parameters which can beused in applying control functions are instant and historicaltemperature data for the external surface of the receiver, and instantand historical light energy flux density data for the external surfaceof the receiver. For example, the distribution of temperature across thesurface of a receiver at a given moment can be compared with apre-determined set of desired values or with the data for an earliermoment in time in order for the controller to decide whether currentheliostat aiming instructions are adequate to meet system optimizationgoals or safety-based operational constraints, and especially whentaking into account measured and predictive weather data. Similarly, thedistribution of light energy flux density across the surface of a targetat a given moment can be compared with a predetermined set of desiredvalues, or, alternatively, used to calibrate the calculation ofpredicted flux densities that are used by a control system whichgenerates sets of aiming points and directs heliostats to those aimingpoints based on those predicted patterns of resultant light energy fluxdensity. The skilled artisan is directed, for example, WO/2009/103077incorporated herein by reference in its entirety.

Another function of a control system includes the calibration ofheliostats, or more specifically, the calibration of the reflection ofsolar radiation on a target with respect to a desired or predictedreflection, for example in terms of the location of the reflection, orin terms of the shape of the reflection, or in terms of the intensity oflight flux at a plurality of points in the reflection, or in terms ofany combination of data that describes the beam projection (reflection)in a desired format. As noted above, this functionality may be providedby the heliostat controller of a single heliostat either autonomously orin response to electronic communications received, for example, from aheliostat field controller.

In some embodiments, a system for calibrating heliostats includes anarray of light intensity detectors. FIGS. 4A-4E, 6A-6B, 7A-7B, 13illustrate various arrays 100 of light intensity detectors 101. In onenon-limiting example, (see, for example, FIGS. 8-10, 12) the array 100of light detectors 101 is mounted on tower 43, for example, below theheliostat target.

In some embodiments, at one or more times, instead of being directed atthe target, the reflection beam 398 (i.e., a reflection of a beam ofsunlight or artificial light incident to mirror 8) produced by eachheliostat may be directed at array 100 of light intensity detectors.Each light intensity detector is used for detecting the intensity oflight reflected by heliostats. In the figures, element 399 representsthe cross section of reflection beam 398 as it is projected onto thearray 101 of light sensors—in various embodiments, a maximum dimensionof the cross section 399 of reflection beam 398 as projected onto thetarget and/or the array 101 of light intensity may be at least 30 cm, orat least 70 cm or at least 1 meter or at least 1.5 meters.

As will be discussed in greater detail below, light intensity dataacquired by each of the light intensity sensors may be used tocharacterize reflection beam 398 and/or a property of the cross-section399 thereof to determine a ‘projected beam property.’ In onenon-limiting example, a measurement of the shape or cross-sectional-area(or an indicative parameter thereof) may be derived from the lightintensity data. In another example, a beam intensity map measuring theflux intensity at different locations of the reflected beam crosssection 399 may be derived from the light intensity data. In yet anotherexample, a so-called beam offset may be derived from the light intensitydata (see the discussion below with reference to FIG. 8).

The light intensity data and/or data may be useful calibrating theheliostat to determine and/or modify one or more operating parameters ofone or more of heliostats 38. The heliostat calibration may be carriedout in a closed-loop system although alternatively it can be used in aopen-loop system. A closed-loop system is one in which the data obtainedor derived by the light intensity detector array is used to changeheliostat aiming instructions, to change the characterization of aheliostat in a database, or to bring about heliostat maintenance byhaving a computer program analyze the data and issue electronicinstructions on a periodic or real-time basis without significantoperator intervention. An open-loop system is one in which the data isstored or analyzed, and used at a later time for changing heliostataiming instructions or for bringing about heliostat maintenance, usuallyafter intervention by a human operator.

Light intensity detectors can include image sensors using charge-coupleddevice (CCD) or complementary metal-oxide-semiconductor (CMOS)technology, or devices incorporating such a sensor. For example, aconsumer digital camera with a CCD ‘chip’ can serve as an image sensor.Light intensity detectors can also include photodetectors, so-calledlight meters, which generally include a photovoltaic or photoresistantsensor. Alternatively, as is known in the art, ordinary solar cells suchas photovoltaic or photoelectrochemical cells (for example, inefficientphotovoltaic or photoelectrochemical cells) can be used as lightintensity detectors. According to some embodiments, any one of thesedevices can be used to register a digital representation of the lightreflected by a heliostat mirror 8 from a light source, either viadigital imaging or by direct or indirect registration of light intensitylevels.

In some embodiments, the array 100 of light intensity detectors 101 ispreferably positioned so as to be accessible to the reflected lightbeams of large numbers of heliostats and therefore are best located at,near or on a central tower on which a receiver or other target islocated since large numbers of heliostats are generally capable ofaiming reflected light in the direction of a central tower. The array100 is most preferably close to a target (such as a receiver orsecondary reflector) so as to minimize travel time of heliostatsdiverted from regular tracking (focusing reflected light onto thetarget) for the purposes of calibration (see for example, FIGS. 8-10,12).

FIGS. 4A-4E are illustrations of a one dimensional array 100 of lightintensity detectors 101. FIGS. 6A-7B and 13-14F are illustrations of atwo dimensional array 100 of light intensity detectors 101. In theexample of the figures, the array of light intensity detectors are‘macro-arrays’ of light intensity detectors where the maximumsensor-sensor distance between light-intensity sensors of the array is(i) at least 0.5 meter and/or (ii) at least 0.5 times or at least 1.0time or at least 1.5 times or at least 2.0 times the maximum dimensionof the reflected beam's projection 399 onto the macro-array oflight-intensity sensors.

The “maximum sensor-sensor distance” is the maximum distance between anypair of sensors of the macro-array 100 of sensors 101—in the example ofFIGS. 4A-4E the maximum sensor-sensor distance is the distance between101A and 101H, in FIGS. 14A-14F the maximum sensor-sensor distance isthe distance between 101A and 101G. In FIG. 4A, the maximum dimension ofthe reflected beam's 398 projection 399 is indicated by 397.

FIGS. 4A-4F indicate a plurality of snapshots in time as the reflectedbeam's 398 projection 99 beam projection traverses the macro-array oflight-intensity sensors. In the example of FIGS. 4A-4F, light intensityreadings at each of the sensors 101A-101H may be recorded for aplurality of points in time t0-t4. The shape of the beam (or any otherbeam projection parameter) may be determined according to: (i) the timeseries of the light intensity measurements; (ii) the speed at which theprojection beam traverses the macro-array of light intensity sensors(this may be constant or may vary in team); and/or (iii) the distancebetween the various sensors 101 at which light intensity measurementsare taken.

Thus, in one example related to FIGS. 4A-4F, the computed area of thereflection beam 399 may be a function of the distance between 101B and101G (where a large distance would indicate a larger area). Furthermore,in the example of FIGS. 4A-4F, the computed area of the reflection beam399 may be a function of the reflected beam's projection traversalspeed. In this case, a faster speed may indicate a larger beam area forfixed points in time, a larger speed indicates that the projection ofthe reflection beam has traveled a greater distance.

Thus, in some embodiments, the system is capable of measuring orapproximating the shape of a heliostat beam projection from time seriesdata of light intensity detectors, including data obtained from movingheliostats. This is best accomplished by designing the size and shape ofthe array of light intensity detectors that can do this in conjunctionwith the movement of a heliostat. For example, as illustrated in FIGS.4A-4F, the array 100 comprises a single line (or alternatively arc, notshown) of light intensity detectors 101, provided substantiallytransversely to the tracking path of the beam projection 399 of lightreflected from a heliostat, may be used to generate a set of time seriesthat can be used by the system, together with ‘external’ data on thetracking speed of a heliostat and optionally the distance of a heliostatfrom the array, to approximate the shape of a heliostat beam projection.In another example, illustrated in FIG. 5A, an array 100 includes atleast one additional and optionally parallel line or arc of lightintensity detectors 101, which can be added in order to facilitatemeasurement or approximation of the speed of the beam projection 399(since the distance between any two detectors 101 in the path of thebeam projection 399, for example 101 b and 101 a in the figure, could bemade known to the system). In yet another example, illustrated in FIG.5B, the array 100 includes a two-dimensional array, or matrix, of atleast partly offset rows or columns of detectors 101 that serve toincrease resolution, in one or two dimensions, of the detection ormeasurement of light intensity. The size of the matrix can becontinually increased to improve the resolution of the data capture, butultimately the decision on the size of the matrix will be based on aneconomic tradeoff between the cost of additional light intensitydetectors versus the incremental added value of higher resolution in theraw data. The incremental added value may also depend on the size of aheliostat beam projection and the number of heliostats. For example, ifthe detectors are expensive, and the deployment of a very large numberof heliostats (tens of thousands) allows the achievement of only amoderate level of precision in calibration, then a system designer mayprefer to use a small number of detectors (say, fewer than ten), whereasif the detectors were to be inexpensive and a smaller number of largerheliostats were to be involved which would typically demand a higherlevel of calibration accuracy, then a system designer may choose to usea larger number of detectors. In yet a further example, illustrated inFIGS. 6 a and 6 b, an array 100 of light intensity detectors 101 may bearranged with different and non-uniform densities of detectors 101 indifferent areas of the array 100, for example so as to provide higherresolution at the edges of the array 100 (FIG. 6 a), or alternatively soas to provide higher resolution at the center of the array 100 (FIG. 6b). Such non-uniform placement of detectors 101 may be, for example, inorder to obtain a projection perimeter with greater resolution (as inFIG. 6 a) or for the purpose of determining with greater precision thestatistical distribution or even the calculated centroid of a heliostatbeam projection 399 (as in FIG. 6 b).

In FIGS. 4A-4E (and in other figures) the light intensity detectors areseparated by a distance on the order of magnitude of the size of thecells—however, this is not a limitation, and larger or smallerseparations are certainly possible. In some embodiments, larger orsmaller separations are certainly possible. as long as the maximumseparation is on the order of magnitude of the size of the team. In onenon-limiting embodiment, all of the light detectors are part of a largePVC panel having inefficient PVC cells.

FIGS. 7A-7C are flow charts of routines for operating a heliostat of asolar energy system according to some embodiments.

Reference is made to FIG. 7A. In step S301 a heliostat is directed atthe target (e.g. a solar energy conversation target and/or a secondaryreflector) mounted on the tower—for example, to generate energy from thereflected beam that is projected onto the target. In step S305, the sameheliostat is directed to the macro-array 100 such that the projectedheliostat reflection beam traverses across the macro-array of lightsensors.

In a first example, the heliostat is redirected from a first orientationwhen it is aiming at the target to a second orientation when to aim atthe macro-array of light sensors (and traverse across the macro-array).In a second example, the heliostat is directed at the target, and thenmay be re-directed to aim away from both the tower and the macro-arrayaltogether—according to this second example, only at after some timedelay. (e.g. a time delay of minutes, hours or days between the timewhen step S301 finishes and the time when step S305 begins). In stepS309 the light intensities are measured by light intensity sensors 101of macro-array 100. In step S315, one or more beam projection parametersof the heliostat are determined.

Reference is now made to FIG. 7C. In step S309, for each heliostat,respective light intensities are measured. In step S361, the lightintensity data is analyzed. In one particular example, the respectiveshape or flux intensity map of each heliostat is determined—for example,to create a database of heliostat shapes or heliostat intensity maps. Instep S365, according go the results of the data analysis, heliostatselection may be carried out—i.e., a sub-plurality of the plurality ofheliostats may be selected for simultaneous aiming at the target. In oneexample, it may be desired to provide a certain flux distribution at thetarget, and heliostat reflection beams (whose beam parameters are knownfrom the light intensity data) may be selected accordingly.

FIG. 8 illustrates the concept of heliostat offset. It is noted that inmany cases, heliostat controller attempts to aim the heliostat at thetarget so that the centroid of the reflection beam is located at targetcentroid location 660. In many real-world scenarios, over time certainfactors may cause the heliostat to deviate from its preferred operatingparameters—for example, wind or rain may move the mirror or one or moreheliostat moving parts associated with the aiming the heliostat, changesin temperature may distort the mirror, seismic activity may influenceheliostat aiming or any other factors may influence heliostat aiming.

For the present disclosure, the terms ‘aiming’ and ‘directing’ are usedinterchangeably.

Thus, as illustrated in FIG. 8, the actual centroid location 664 of thereflected heliostat beam obtained when the heliostat controller attemptsto aim at location 660 actually deviates from the target centroidlocation

Embodiments of the present invention provide techniques and apparatusfor measuring the actual centroid location 664 according to the lightintensity measurements detected by light intensity sensors 101 of themacro-array. If desired, this information may then be used to determinethe offset vector 668.

Reference is now made to FIG. 7B. In step S301*, the heliostat isdirected to the target mounted on the tower (i.e., an solar energyconversion target and/or a secondary reflector) according to an initialset of aiming parameters. Steps S305 and S309 of FIG. 7B is the same asstep as steps S305 and S309 of FIG. 7A.

In step S321, the heliostat is directed to the target according to amodified set of aiming parameters. Thus, in one non-limiting use case,it is possible to compute the actual centroid location 664 and/or offsetvector 668 and then to configure the heliostat control to compensate forany offset. In one particular example, it is possible to immediatelyre-aim the heliostat at the target according to the modified set ofaiming parameters. Alternatively, it is possible to only do so after atime delay.

In another example, it is possible (i) at an initial time t1 to directthe heliostat in step S301* according initial parameters at the targetaccording to an initial set of aiming parameters; (ii) at a later timet2 (either immediately after S301* or after any time delay—it isappreciated that in the interim the heliostat may be directed in anydirection including away from both the target and the macro-array in theinterim) to direct the heliostat in step S305 at the macro-array oflight-intensity sensors where in step S309 the light intensitiesmeasurements are taken and (iii) at yet a later time t3 (eitherimmediately after S305 or after any time delay—it is appreciated that inthe interim the heliostat may be directed in any direction includingaway from both the target and the macro-array in the interim) in stepS321 to direct the heliostat back at the target according to a modifiedset of aiming parameters.

In some embodiments, the system includes software for providinginstructions to heliostats to track to the array, including at least oneset of tracking coordinates and tracking speed. The instructions can bepropagated through a data network or communicated directly in accordancewith the architecture of the solar field control system. Theinstructions, if transmitted in advance, may include a time when theheliostat controller should initiate execution of the instructions, andthe heliostat controller may be equipped with data storage means forstoring such instructions. Alternatively the instructions can bepre-programmed in a heliostat controller. For example, a heliostatcontroller may include a stored set of instructions to track to thecalibration array with a given periodicity such as, for example, weeklyor monthly.

In preferred embodiments, the heliostat calibration system is capable ofobtaining a time series of data points representing the light intensityreflected by a heliostat to each digital imaging device or other lightintensity detector, including while the heliostat is in motion. Forexample, if it takes 30 seconds for the light reflected from a heliostatin motion to traverse a light intensity detector while the heliostat istracking across the detector, then the time series would include aplurality of data points (digital images and/or digital light intensitymeasurements) captured during those 30 seconds and preferably at aresolution sufficiently high as to indicate with a desired level ofprecision the beginning and end of the incidence of light on thedetector as well as the intensity level at each time point. In anotherexample, all of the detectors in an array may capture a time series ofdata points beginning when a first detector in the array detects lightreflected from a heliostat and ending only when no detectors in thearray detect reflected light from the heliostat. In yet another example,all of the detectors in the array obtain, record or process lightintensity data (or digital images) all of the time when it is known thatheliostats are to be calibrated, leaving the task of determiningbeginning and ending time points for each individual heliostat to image-or data-processing software elsewhere in the system. The time seriesdata from each light intensity detector can be recorded for laterprocessing, and/or transmitted, whether directly or through a datanetwork, to a computer or data storage device elsewhere in the systemfor processing and analysis of the data.

In particularly preferred embodiments, the system also includes computerhardware and software for analyzing the data obtained or recorded fromthe digital imaging devices or other light intensity detectors. Theanalysis is performed for the purpose of calibrating the heliostat,where calibrating may include at least one of: determining the deviationof the calculated centroid of the heliostat's beam projection from thepredicted; determining or approximating the beam projection shape andits deviation from the predicted; determining the intensity of light ata plurality of points within the beam projection and any deviation fromthe projected distribution of light intensity; determining the speed ofthe traversal of the beam projection and any deviation from thepredicted; correcting a structural or assembly error, or shapeaberration, or any other malfunction or deviation from design in aheliostat; storing or using any of these data elements for the purposeof updating or changing a database of heliostat-related data or ofupdating or changing the aiming and/or tracking instructions of aheliostat; or analysis of the data by a system designer or operator.

Most preferably, the analysis software is capable of calculating a beamprojection shape and/or calculating the statistical distribution and/orcentroid of the beam projection distribution, using data obtained and/orrecorded by the light intensity detectors, including time series data,and optionally using statistical techniques applying a Gaussian or otherprobabilistic distribution to the light intensity of a heliostat beamprojection. Additionally, the software can be capable of producing adigital map of the light intensity at a plurality of points in the beamprojection. Any of these calculated parameters can be used in thecalibration of heliostats as described above. Heliostats (or a controlsystem for heliostats and/or heliostat controllers) are configured tomodify aiming instructions such as target coordinates in response todata obtained during the calibration process or in response to theresult of the analysis of the data.

The analysis software can also include software to eliminate or cancelout the effects of diffuse or ambient light measured by a lightintensity detector, for example by measuring such light before and/orafter the traversal of a heliostat beam. The analysis software can alsoinclude software for transformation of a curvilinear projection in orderto ‘translate’ a beam projection shape and/or map of light intensityvalues to the surface geometry of a receiver, taking into account: thedifferent angle of incidence of reflected light on the receiver comparedwith that on the array; the different attitude of the receiver withrespect to the heliostat field; and/or the external surfacecharacteristics of the receiver (for example which may compriseindividual round boiler tubes rather than a smooth external surfacepanel.

In another embodiment, a solar power tower system includes a solar fieldand an array of light intensity detectors on a tower. A target such as athermal or photovoltaic receiver, or alternatively a secondaryreflector, is situated at or near the top of the tower. The array oflight intensity detectors can be provided in accordance with any of theembodiments described above.

In the case of a thermal receiver or photovoltaic target, for example,the array of detectors would optimally be provided just below thereceiver on the side of the tower as shown in FIG. 9A. Referring now toFIG. 9A, a receiver 1 sits atop a tower 43, similar to the arrangementof FIG. 1. At least one heliostat 38 is configured with tracking andpivoting means as described above to reflect sunlight 28 onto thereceiver 1. An array 100 of light intensity detectors (not shownindividually) is positioned on the tower 43 below the receiver 1 so thata heliostat 38 can also track to the array 100 and reflect sunlight 28onto the array 100. Reflected light 30 falls upon the array inaccordance with tracking instructions executed by a heliostat 38 fromtime to time. The array 100 can optionally be angled toward the solarfield in order to cause light reflected from heliostats to impinge uponthe array at a more desirable angle, if the benefits of such an anglingwould outweigh additional material, installation and/or maintenancecosts.

In the case of a secondary reflector used as a ‘beam-down’ mirror, thearray of detectors could be either above the secondary reflector or onone of the tower supports of the secondary reflector as shown in FIG.9B. In FIG. 9B, receiver 1 sits on—but not at the top of—a tower 43 (oralternatively near or at its base) on which is provided a secondaryreflector 9, similar to the arrangement of FIG. 2. At least oneheliostat 38 is configured with tracking and pivoting means as describedabove to reflect sunlight onto the secondary reflector 9. An array 100 aof light intensity detectors (not shown individually) is positioned onthe tower 43 below the secondary reflector 9 so that a heliostat 38 canalso track to the array 100 b. Alternatively, (or, optionally,additionally,) an array 100 b of light intensity detectors (not shownindividually) is positioned on the tower 43 higher than the secondaryreflector 9, in a location allowing a heliostat to track to the array100 b without the beam being blocked by the receiver at least part ofthe time.

In a preferred embodiment, the array of light intensity detectorsincludes optical or mechanical elements to improve the ability of thedetectors to detect or measure the light reflected by heliostats. Anexample of a mechanical element is one that substantially blocks directsunlight, or sunlight reflected by objects other than heliostat mirrors,from reaching the detectors during most of the hours of the day, such asa shade or awning. Such an element can also serve to keep precipitationand some windborne particles off the light intensity detectors. FIG. 10shows an example of a mechanical element 105 positioned on a tower 43 soas to reduce direct sunlight and/or precipitation impinging on an array100 of light intensity detectors (not shown individually).

An example of an optical element is a filter that can be placed overindividual light intensity detectors, or alternatively over the entirearray or a portion thereof, in order to reduce total or maximum lightintensity to a level more appropriate to the sensitivity or operatingcharacteristics of the light intensity detectors. Other examples ofoptical elements may include lenses with anti-reflective coatings ordust-repellent coatings, focusing lenses or spectrally selectivefilters. Alternatively, light intensity may be moderated by software.

In a particularly preferred embodiment, the solar power tower systemincludes multiple arrays of light intensity detectors in order to makesuch arrays accessible to all the heliostats in a solar field. In anexample, a solar power tower system includes a surround receiver on afour-sided tower, and additionally includes a surround field ofheliostats, i.e., 360° around the tower. In this case the system wouldinclude four arrays, one on each side of the tower. As the lightintensity detectors should be selected to allow for an acceptance anglewide enough to accommodate the respective portion of the solar field, inthis example the acceptance angle (for each of four arrays) would haveto be 90°, as illustrated in FIG. 11. Referring now to FIG. 11, a tower43—on which a receiver (not shown) is sited—is surrounded by a solarfield 39 comprising, inter alia, a plurality of heliostats 38 in each offour quadrants Q1, Q2, Q3 and Q4. Light intensity detector arrays 100,each with an acceptance angle of 90°, are positioned on the tower suchthat each array 100 can accommodate calibration of the heliostats in oneof the four quadrants Q1, Q2, Q3 and Q4.

In another preferred embodiment, the solar power tower system includes alight projector that can be used for performing heliostat calibration atnight. The projector must be of sufficient power as to allow the lightintensity detectors to register the intensity of its light whenreflected thereupon by a heliostat, with a desired level of dataresolution. An example of a suitable light projector is a StrongBritelight® 10000 available from Ballantyne of Omaha, Inc., of Omaha,Nebr., although it is possible to use a light projector of lower powerrating as well. Operation of such an embodiment is illustrated in FIG.12, where a light projector 108 mounted on a tower 43 between a receiver1 and a light intensity detector array 100 shines light 31 onto aheliostat 38, and reflected light 32 strikes the light intensitydetector array 100 in accordance with tracking instructions executed bythe heliostat 38 from time to time. In an alternative preferredembodiment, the projector will be designed so as to create a source oflight (as seen from the solar field) of a size comparable to theapparent size of the disk of the sun (for example 9 mrad as ‘seen’ by aheliostat), in order to allow for registration and calibration of notonly aiming and tracking accuracy but also of beam projection shapedeviation from a desired or predicted shape.

In other embodiments, a method for operating a solar power tower systemincludes using an array of digital imaging devices or other lightintensity detectors to capture and/or record the light reflected from aheliostat for the purposes of calibration, where calibration can includeat least one of: determining or approximating a statistical distributionand/or centroid of a heliostat's beam project and/or its deviation froma desired or predicted set of values; determining the beam projectionshape and/or its deviation from a desired or predicted set of values;determining the intensity of light at a plurality of points within thebeam projection and/or any deviation from a desired or predicted set ofvalues; determining the speed of the traversal of the beam projectionand/or any deviation from a desired or predicted set of values;correcting a structural or assembly error, or shape aberration, or anyother malfunction or deviation from design in a heliostat; storing orusing any of these data elements for the purpose of updating or changinga database of heliostat-related data or of updating or changing theaiming and/or tracking instructions of a heliostat; or analysis of thedata by a system designer or operator. According to the method, thearray is used for calibration of heliostats in a solar power towersystem by causing each heliostat, or alternatively groups of heliostats,to traverse the array periodically in accordance with a manufacturer'sspecification, for example once every two weeks, once every month, oronce every two months. Therefore, the method preferably includes sendinginstructions, directly or through a data communications network, to aheliostat to cause it to track to the array. Alternatively it would bepossible to make use of a preprogrammed heliostat controller whichcauses a heliostat to track to the array with a desired periodicity orunder certain preset conditions. In any of the embodiments, lightreflected by the heliostat onto an array of light intensity detectorscan come from the sun, the moon, or from a light projector.

The method also preferably includes selecting heliostats for tracking tothe light intensity detector array in accordance with their relativeavailability or, conversely, with in accordance with how much eachheliostat is needed by the solar power tower system. For example, it isknown that during hours of peak insolation many heliostats are turnedaway from their usual receiver or other target in order not to overloada receiver or some other system component (such as a turbine in the caseof a concentrated solar thermal plant, or power inverters in the case ofa concentrated photovoltaic plant), or so as not to exceed a contractualor regulatory limit (for example the conditions of a power purchasingagreement). It is therefore desirable to select those heliostats notinstantly required during such peak insolation hours, and instead tocause them to track to the calibration array at that time. In anotherexample, there may be excess heliostats on one side of a tower; forexample, it is known that the heliostats east of a tower in theafternoon (in the northern hemisphere) can reflect up to three times asmuch light onto the eastern side of a receiver than they can in themorning (because reflected light is reduced in accordance with thecosine of half the angle between incidence and reflection). Inaccordance with the method it would be desirable to cause such excessheliostats to track to the calibration array during such times as theyare not needed for energy conversion so as not to make them unavailableat other times when they are more acutely needed (e.g., the morninghours in the eastern field example).

The method also preferably includes obtaining and optionally recording atime series of the light intensity reflected by the heliostat to eachdigital imaging device or light intensity detector, including while theheliostat is moving. If the tracking speed of the heliostat and thedistance from the heliostat to the array are known, then it is possibleto calculate at least one dimension of the beam projection from the timeseries; alternatively, by using a known distance between members of thearray, it is possible to calculate at least one dimension of the beamprojection from the time series without the need for external data.

The method most preferably includes analyzing data obtained and/orrecorded from the array of light intensity detectors (or digital imagingdevices) to yield a characterization of the beam projection of aheliostat, where the characterization includes at least one of: a map ofthe light intensity at a plurality of points in the beam projection; theshape of the beam projection either as a set of points describing aperimeter or a mathematical expression for the shape; a mathematicalexpression for distribution of light in the beam such as a statisticaldistribution; a beam centroid; or the deviation of any of these measuredor characterized parameters from a design target or from a predicted setof values. According to the method, the characterization, optionallyincluding any measurable or calculable deviation from a design goal orpredicted set of values, is optimally used by a control system and/orsystem operator to calibrate the aiming of the heliostat or for anyother aspect of heliostat calibration as described above.

In a preferred embodiment, the method includes causing a plurality ofheliostats to track simultaneously or nearly simultaneously to an array,and obtaining (or recording) light intensity indications for thepurposes of heliostat calibration. In an example illustrated in FIG. 13,four heliostats track simultaneously to a two-dimensional array (i.e.,an array having at least two columns and two rows, where the columnsand/or rows can be in the shape of lines, staggered lines or arcs) fromdifferent directions at the same time in such a way that each heliostatbeam projection can be independently analyzed. This arrangement isoptimally arranged so that at least part and preferably at least half ofeach beam projection 399 traverses at least one row 106 or column 107 oflight intensity detectors 101 within the array 100 before intersectingor overlapping with another beam projection 399. Similarly, afterintersecting with other beam projections 399, at least part andpreferably at least half of each beam projection 399 traverses at leastone row 106 or column 107 of light intensity detectors 101 within thearray 100 after ceasing to intersect or overlap with other beamprojections 399. In the example, software captures most or all of thedesired beam shape times series data for each beam projection during thetime that the beam projection doesn't intersect with other beamprojections. In another example, the light intensity detectors aredigital imaging devices and different pixels or groups of pixels in theimaging sensor can be used for recording the light intensity ofdifferent heliostats.

FIGS. 14A-14F illustrate time series of multiple beams 99A, 99Btraversing an array 100 of light intensity sensors 101 in accordancewith some embodiments. At times t0, t1, t2 and t5 beams 99A and 99B donot overlap (they are disjoint)—at times t3 and t4 beams 99 a and 99Boverlap.

FIG. 15A is a flow chart of an exemplary routine for determining one ormore beam projection parameters of a heliostat in situations wheremultiple heliostat reflection beams are simultaneously incident on themacro-array 100 of light intensity detectors 101 so that multiplereflection beams overlap at one or more light intensity detectors 101.

In step S501, a plurality of heilostats are controlled so that multiplereflection beams simultaneously traverse the macro-array of lightdetectors.

In step S555, when the projections of two or more heliostat reflectionbeams 398 simultaneously traverse the macro-array 100 of detectors 101,one or more sensors are simultaneously illuminated by multiplereflection beams (for example, in FIG. 14D sensors 101C and 101F aresimultaneously illuminated by beams whose projection is 99A and 99B; inFIG. 14E sensors 101G and 101F are simultaneously illuminated by beamswhose projection is 99A and 99B).

According to some embodiments, when multiple beams reflected from aheliostat simultaneously illuminate a given light intensity sensor 101,it is possible to determine the light intensity contribution of eachreflection beam of multiple beams.

In one non-limiting example, the light intensity detectors are imagedetectors, and a respective image is acquired by each light intensitydetector. It is possible to analyze the contents of each image and inaccordance with the results of the image analysis, to determine therelative contributions.

FIG. 15B illustrates a plurality of heliostats 38A, 38B configured tosimultaneously reflect respective incident beams 28A, 28B onto amacro-array 100 of image detectors which provide light-intensitydetector functionality.

When simultaneous reflection beams 98A, 98B are incident upon an imagedetector (i.e., overlapping beam projections as in FIGS. 14D-14E), theimage generated may, in one example, look like the image illustrated inFIG. 15C. In FIG. 15C, the image is at least a portion of the field ofheliostats from the point of view of the one of the image sensors forthe particular case of a light intensity sensor looking down upon thefield of heliostats. The field of heliostats includes two heliostats38A, 38B (having mirrors 8A and 8B) simultaneously directed to a givenimage detector to simultaneously illuminate the given image detector. Inthe example of FIG. 15C, for illustrative purposes only, many of theheliostats are drawn identically—it is appreciated that in manyapplications, this typically is not the case, and heliostat mirrors mayhave different shapes and/or sizes and/or orientations.

It is now disclosed that it is possible to analyze the image acquiredthe image by the image detector, and according to the contents todetermine (within some tolerance) the relative contributions of multipleheliostat reflection beams reflected onto the image detector. Thus, inthe example of FIG. 15C, the contribution of heliostat 38A (38B) havingmirror 8A (8B) is a function of the size in the image of heliostat 38A(38B) (where a larger size would indicate a larger light contribution)and the color and/or grey shade of heliostat 38A (38B). Because the greyshade (or color) of the heliostat may be indicative of the intensity ofthe incident light beam 28 (and hence the reflected light beam 398) on aheliostat, this may be useful for determining a relative contribution oflight intensity of a particular heliostat. In addition, the size of theheliostat in the image may also be useful for determining a relativecontribution of light intensity of a particular heliostat.

In some embodiments, towards this end, it may be useful to maintain adatabase of heliostats shapes and sizes in order to identify theparticular heliostats in the image.

Thus, in the embodiment of FIG. 15A, it is possible to determinerelative contributions to the light intensity of reflection beams ofdifferent heliostats. Alternatively or additionally, as in FIG. 16, itis possible to give a greater weight (when computing a heliostatprojected beam parameter such as beam shape or centroid location orintensity or when determining modified operating parameters for one ormore heliostats) to images acquired when the beams are not overlapping,For example, in some embodiments, it may be assumed that themeasurements from a single heliostat beam are more reliable thanmeasurements taken from a light detector simultaneously illuminated bymultiple heliostat reflection beams.

Thus, in step S501 of FIG. 501, the plurality of reflection beamstraverse the macro-array of light detectors.

In steps S505-S509, it is determined if multiple reflection beams aresimultaneously illuminating a light intensity detector—for the exampleof FIGS. 14A-14F, for detector 101F this is true at time=t3 and time=t4,for detector 101A this is never true, for detector 101 C this is trueonly for time=t3.

In the event of an overlap (step S521) then the data acquired at a lightintensity sensor simultaneously illuminated by multiple beams isdiscarded or is given a lower weight. In the event of no overlap (stepS513), then a decision to utilize this data when characterizing the beammay be made and/or the data may be given a greater weight—for example,when computing or more beam projection parameters or when effecting adecision based upon the measurements of one or more light intensitydetectors.

In addition to the techniques discussed above, multiple beamssimultaneously incident on the macroarray may be numerically separatedfrom multiple time samples of the overlapping beam images. This may bedone using known image processing techniques. Further, images can becaptured at higher resolution that a sparse detector array bydisambiguating multiple sparse images from a time series.

Certain features of this invention may sometimes be used to advantagewithout a corresponding use of the other features. While a specificembodiment of the invention has been shown and described in detail toillustrate the application of the principles of the invention, it willbe understood that the invention may be embodied otherwise withoutdeparting from such principles.

1. A solar energy system comprising: a. a plurality of heliostatsconfigured to reflect sunlight to a target mounted on a tower, eachheliostat including a respective heliostat controller, the target, thetarget being selecting from the group consisting of an energy conversiontarget and/or a secondary reflector; and b. a macro-array oflight-intensity sensors characterized by a maximum sensor-sensordistance and mounted on the tower such that when any heliostat of theplurality of heliostats reflects a beam of light onto the macro-array oflight-intensity sensors, the maximum dimension of the reflected beam'sprojection on the macro-array is at most twice the maximum sensor-sensordistance, wherein each heliostat controller is operative to control itsrespective heliostat so that the light beam reflected by the heliostattraverses the macro-array of light-intensity sensors.
 2. The system ofclaim 1 wherein the macro-array of light-intensity sensors issubstantially co-planar.
 3. The system of claim 1, where the macro-arrayof light-intensity sensors is a two dimensional macro-array.
 4. Thesystem of claim 1, where the light-intensity sensors are configured toacquire time-series light intensity data while the reflected beam'sprojection traverses across the macro-array of light sensors.
 5. Thesystem of claim 1 wherein the heliostat controller is operative to: i)before the traversing of the projection of the reflection beam, directthe heliostat to the target mounted on the tower according to an initialset of aiming parameters; and ii) after the traversing of the projectionof the reflection beam, re-direct the heliostat to the target mounted onthe tower according to a modified set of aiming parameters that ismodified in accordance with light intensity data generated by lightintensity sensors of the macro-array.
 6. The system of claim 5 whereinthe heliostat controller is operative to effect the re-directingaccording to the modified set of aiming parameters after the beamtraversing.
 7. The system of claim 5 wherein the heliostat controller isoperative to effect the re-directing according to the modified set ofaiming parameters immediately after the beam traversing.
 8. The systemof claim 4 wherein the heliostat controller is operative to effect there-directing according to the modified set of aiming parametersimmediately only after a time delay.
 9. The system of claim 8 whereinduring the period of the time delay, the controller is operative tore-direct the heliostat to the target according to the initial set ofaiming parameters.
 10. The system of claim 6 wherein the modified set ofaiming parameters is modified in accordance with at least one of: i)distances between light-intensity sensors of the macro-array oflight-intensity sensors; and ii) a beam traversal speed of thetraversing reflected heliostat beam.
 11. The system of claim 1 furthercomprising: c. a heliostat-field controller operative to: i) select,from the plurality of heliostats, a sub-plurality of heliostats that isto be simultaneously directed to the target; and ii) direct the selectedsub-plurality of heliostats the target, wherein the heliostat fieldcontroller is operative to carry out the heliostat selection inaccordance with respective light intensity measurements of macro-arraytaken when each heliostat's reflected beam respectively traverses themacro-array.
 12. The system of claim 11 wherein the heliostat-fieldcontroller is operative to effect the selection in accordance with atleast one of: i) distances between light-intensity sensors of themacro-array of light-intensity sensors; and ii) a beam traversal speedof the traversing reflected heliostat beam.
 13. The system of claim 1wherein the system further comprises: c) electronic circuitry configuredto measure at least one beam projection parameter of the heliostat beamaccording to the light intensity measurements acquired bylight-intensity sensors while the heliostat beam traverses themacro-array of light-intensity sensors.
 14. The system of claim 13wherein the electronic circuitry is configured to effect the measuringin accordance with at least one of: i) distances between light-intensitysensors of the macro-array of light-intensity sensors; and ii) a beamtraversal speed of the traversing reflected heliostat beam.
 15. Thesystem of claim 1 wherein the system further comprises: c) electroniccircuitry configured to measure at least one of: i) a shape of theheliostat beam; ii) a flux intensity map of the heliostat beam; iii) anoffset of the heliostat beam; and iv) an indication of beam area.according to the light intensity measurements acquired bylight-intensity sensors while the heliostat beam traverses themacro-array of light-intensity sensors.
 16. The system of claim 15wherein the electronic circuitry is configured to effect the measuringin accordance with at least one of: i) distances between light-intensitysensors of the macro-array of light-intensity sensors; and ii) a beamtraversal speed of the traversing reflected heliostat beam.
 17. Thesystem of claim 1 wherein: the heliostat controllers collectively areconfigured so that multiple overlapping heliostat reflection beamsincluding first and second heliostat reflection beams simultaneouslytraverse the macro-array to simultaneously illuminate one or more of thelight-intensity sensors.
 18. The system of claim 17 wherein: i) thelight-intensity sensors of the macro-array are image sensors; and ii)the system further comprises: c) electronic circuitry operative to: A)determine, from the images generated by the image sensors, relativelight intensity contributions of the overlapping first and secondheliostat beams when the first and second beams overlap and traverse themacro-array; and B) in accordance with the relative light intensitycontributions, determine at least one of: I) a shape of the first and/orsecond heliostat beam; II) a flux intensity map of the first and/orsecond heliostat beam; III) an offset of the first and/or secondheliostat beam; and IV) an indication of beam area.
 19. The system ofclaim 17 wherein the heliostat controllers collectively are configuredso that the first and second heliostat beams overlap at some times andare disjoint at other times while the first and second beams traversethe macro-array.
 20. The system of claim 19 wherein: i) thelight-intensity sensors of the macro-array are image sensors; and ii)the system further comprises: c) electronic circuitry operative todetermine when the first and second beams are disjoint, and inaccordance with the disjoint time period(s), determine at least one of:I) a shape of the first and/or second heliostat beam; II) a fluxintensity map of the first and/or second heliostat beam; Ill) an offsetof the first and/or second heliostat beam; and IV) an indication of beamarea.
 21. The system of claim 1 wherein each of the light sensors of themacro-array are image sensors.
 22. The system of claim 21 wherein theimage sensors are selected from the group consisting of a CCD microarrayand a CMOS microarray.
 23. The system of claim 1 wherein each of thelight sensors of the macro-array are photo-detectors incapable ofdetecting an image.
 24. The system of claim 23 wherein each of the lightsensors are photo-voltaic cells.
 25. The system of claim 1 wherein eachof the light-intensity sensors is mounted to the tower.
 26. The systemof claim 1 wherein the energy conversion target is selected from thegroup consisting of solar boiler target and a molten salt solarreceiver.
 27. The system of claim 26 wherein the solar boiler target isselected from the group consisting of a solar evaporator, a solarre-heater and a solar superheater.
 28. The system of claim 1 wherein theenergy conversion target includes one or more photovoltaic and/orphoto-electrovoltaic cells.
 29. The system of claim 1 wherein a heightof the tower is at least 25 meters.
 30. The system of claim 1 wherein aheight of the tower is at least 100 meters.
 31. The system of claim 1further comprising: c. a projector configured to project artificiallight onto the heliostat such that the traversing reflected beam thattraverses the macro-array includes the artificial light generated by theprojector.
 32. The system of claim 31 wherein the projector is mountedon the tower.
 33. A method of operating a solar energy system, themethod comprising: a. respectively reflecting sunlight from each of aplurality of heliostats to a target mounted on a tower, the target beingselecting from the group consisting of an energy conversion targetand/or a secondary reflector; and b. respectively controlling eachheliostat of the plurality so that the light beam reflected by theheliostat traverses the macro-array of light-intensity sensorscharacterized by a maximum sensor-sensor distance and mounted on thetower such that when any heliostat of the plurality of heliostatsreflects a beam of light onto the macro-array of light-intensitysensors, the maximum dimension of the reflected beam's projection on themacro-array is at most twice the maximum sensor-sensor distance.